NAME

SYNOPSIS

DESCRIPTION

The mlock system call locks into memory the physical pages associated
with the virtual address range starting at addr for len bytes. The
munlock call unlocks pages previously locked by one or more mlock calls.
For both, the addr parameter should be aligned to a multiple of the page
size. If the len parameter is not a multiple of the page size, it will be
rounded up to be so. The entire range must be allocated.
After an mlock call, the indicated pages will cause neither a non-
resident page nor address-translation fault until they are unlocked. They
may still cause protection-violation faults or TLB-miss faults on archi-
tectures with software-managed TLBs. The physical pages remain in memory
until all locked mappings for the pages are removed. Multiple processes
may have the same physical pages locked via their own virtual address
mappings. A single process may likewise have pages multiply locked via
different virtual mappings of the same pages or via nested mlock calls on
the same address range. Unlocking is performed explicitly by munlock or
implicitly by a call to munmap which deallocates the unmapped address
range. Locked mappings are not inherited by the child process after a
fork(2).
Since physical memory is a potentially scarce resource, processes are
limited in how much they can lock down. A single process can mlock the
minimum of a system-wide ``wired pages'' limit and the per-process
RLIMIT_MEMLOCK resource limit.

RETURN VALUES

A return value of 0 indicates that the call succeeded and all pages in
the range have either been locked or unlocked. A return value of -1 indi-
cates an error occurred and the locked status of all pages in the range
remains unchanged. In this case, the global location errno is set to in-
dicate the error.

ERRORS

mlock() will fail if:
[EINVAL] The address given is not page aligned or the length is
negative.
[EAGAIN] Locking the indicated range would exceed either the system
or per-process limit for locked memory.
[ENOMEM] Some portion of the indicated address range is not allocat-
ed. There was an error faulting/mapping a page.
munlock() will fail if:
[EINVAL] The address given is not page aligned or the length is
negative.
[ENOMEM] Some portion of the indicated address range is not allocat-
ed. Some portion of the indicated address range is not
locked.

SEE ALSO

HISTORY

The mlock() and munlock() functions first appeared in 4.4BSD.

BUGS

Unlike The Sun implementation, multiple mlock calls on the same address
range require the corresponding number of munlock calls to actually un-
lock the pages, i.e., mlock nests. This should be considered a conse-
quence of the implementation and not a feature.
The per-process resource limit is a limit on the amount of virtual memory
locked, while the system-wide limit is for the number of locked physical
pages. Hence a process with two distinct locked mappings of the same phy-
sical page counts as 2 pages against the per-process limit and as only a
single page in the system limit.
MirOS BSD #10-current June 2, 1993 1